The development and refinement of wireless communication services and devices continues to occur at a extremely rapid pace. One problem associated with wireless communication devices relates to determining the physical location of a device. It can be highly desirable to locate a wireless communication device for a variety of purposes, such as when there is reason to believe that a subscriber associated with the device is experiencing an emergency situation, or when the device has been misplaced. A solution to this problem must be carefully considered within the cost, size, and power consumption limitations of wireless communication systems and devices. The problem is further compounded when the wireless communication device is located in an obstructed area, such as inside a building.
One proposed solution for determining the location of a wireless communication device is to use the wireless communication device as a transponder, the device echoing back a location-determining signal to multiple base stations. The delay in the echoed signal is used to determine the distances between the wireless communication device and each of a number of base stations. The location of the device can then be determined from the locations of the base stations and the determined distances. However, it is difficult to measure the echoed signal delay with sufficient accuracy, particularly when the wireless communication device is located in an obstructed area.
A second proposed solution is based on the well-known Global Positioning Satellite (GPS) system, and involves incorporating a GPS receiver into the wireless communication device. In the GPS system, a device whose location is to be monitored (in this case, the wireless communication device, but more typically a boat, airplane, truck, etc.) is equipped with a GPS receiver. The GPS receiver can determine its location on the earth's surface, to an accuracy of about 150 feet, based on signals transmitted to the receiver by a GPS satellite. In operation, the GPS receiver receives a time-coded location-determining signal from a first GPS satellite. The receiver determines how long it took the location-determining signal to reach the ground by comparing its departure time (from the satellite) with its arrival time (at the receiver). Based on this time differential, and assuming a transmission speed of, for example, 186,000 miles per second, the receiver calculates the distance to the first satellite (for example, 13,000 miles). Based on preprogrammed information about satellite's orbit, the receiver can determine where in space the satellite is at the time the signal is sent, and the receiver can use this information to determine that its location is somewhere on the surface of a sphere having a radius from the satellite equal to the determined distance (in this example, 13,000 miles). The receiver repeats this process using location-determining signals from additional GPS satellites, and finally determines, based on the points of intersection of the resulting spheres, where it is located.
Unfortunately, a typical GPS receiver is larger than a typical wireless communication device. Even if the GPS receiver is reduced in size, the incorporation of the GPS receiver in a wireless communication device would significantly impact the size of the device. Further, known GPS receivers typically do not have sufficient performance capabilities to determine location within an obstructed area such as inside a building.
Any solution to the problem of determining the location of a wireless communication device which involves significant signal processing operations within the wireless device will drastically affect the device's cost, power consumption, and/or performance.
Therefore, it would be highly desirable to be able to accurately determine the location of a wireless communication device, particularly when the device is located in an obstructed area such as inside a building, in a manner which does not significantly impact the cost, size, performance, or power consumption of the wireless communication device.